Microwaves jiggle (nice technical term that ) all molecules. Thin metal will burn under the effect, as will just about anything else if the heat isn't carried away fast enough by conduction, convection, or radiation, and there is oxygen available.
Sounds very like what you can do with a grape. Cut a garpe almost in half so the two halves are connected by a bit of grape skin. Put that in the microwave for a few seconds and you'll get sparks. So probably not to do with the needles.
I've just done some googling. In the case of grapes, there are various pages mentioning that a grape prepared as above roughly forms a resonant antenna at the frequency used in a microwave oven (quarter wave at 2.45GHz is 30mm, so that sounds about right). Things that are resonant at a frequency can pick up a disproportionate amount of energy from whatever's jiggling at that frequency (think "opera singer breaking wine glass by hitting the right note").
All that sounds quite reasonable as an explanation of why the grape does what it does. I wonder if the gaps between leaf stalks on the stems are around 30mm?